Abstract
Void nucleation, growth, and coalescence at extreme strain rates in ductile metals with weak mechanical anisotropies, e.g., copper, iron, and aluminum, have been extensively investigated. However, the atomic-scale fracture properties of strongly anisotropic metals, especially hexagonal close-packed (HCP) metals, at ultrahigh strain rates have rarely been studied. We have investigated the nucleation, growth, and coalescence of voids in HCP-Zr under isotropic-triaxial tension using molecular dynamics (MD) and void nucleation and growth (NAG) models. The effects of temperature were also examined by MD. The void evolution predicted by MD corresponded to that predicted by the NAG model and is divided into three stages, i.e., an initial nucleation stage, an exponential growth stage, and a linear stage. The nucleation threshold Pn0 is very sensitive to temperature, while the growth threshold Pg0 decreases slightly with increasing temperature. The initial NAG parameters were evaluated by an improved optimized genetic algorithm. In addition, we adjusted the NAG parameters until the history of the void volume fraction calculated by these parameters was exactly the same as that calculated by MD. This study predicts comprehensive NAG parameters for HCP-Zr under extreme conditions, providing a valuable reference for future studies of dynamic damage in HCP materials.
Highlights
Spallation at high strain rates usually occurs in impact or explosively loaded targets
We adjusted the nucleation and growth (NAG) parameters until the history of the void volume fraction calculated by these parameters was exactly the same as that calculated by molecular dynamics (MD)
The NAG model, known as the ductile fracture model (DFRACT) model, developed at Stanford Research Institute, USA, is a microphysical model based on statistical principles that describes the fracture processes that occur as a result of the nucleation and growth of voids in ductile materials
Summary
Spallation at high strain rates usually occurs in impact or explosively loaded targets. In Grady’s study,[1] fracture under high rates of stress loading was viewed as a multiscale problem dominated by a microstructural process through the activation, growth, and coalescence of the interacting cracks. To describe this dynamic fracture process in brittle and ductile metals, Curran et al.[2] first developed the void nucleation and growth (NAG) model at the microscale from statistical principles. Spall strength is dependent on void nucleation and growth under extreme strain rates in ductile metals
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